Entry 16 ======== .. image:: entry16.png :width: 800 Authors ------- - Jan-Pieter Paadekooper About 600 million years after the Big Bang the last major phase transition in the universe took place, in which the neutral hydrogen gas that existed between galaxies was transformed into the hot, ionized plasma it is today. The sources responsible for the ionizing photons that caused this so-called epoch of reionization are still uncertain, but more and more evidence points towards stars in galaxies. However, the contribution of galaxies depends critically on the fraction of the ionizing photons produced by the stars that make it out of the galaxies, the escape fraction. Both observations and numerical simulations have shown that escape fractions are generally very low, thus casting doubt whether galaxies can reionize the universe at all. Our visualisation shows why it is so hard for ionizing photons to escape their host galaxies. We have performed numerical simulations of a large sample of proto-galaxies that formed during the epoch of reionization and post-processed these with detailed radiative transfer simulations to determine the escape fraction. This figure shows on the y-axis the fraction of ionizing photons produced by the stars that reach the outskirts of the galaxies. On the x-axis is the spherically averaged gas column density close to the stellar populations in the galaxy, a direct measure of the number of absorptions in the birth cloud of the stars. The solid line represents the mean in different column density bins, while the colours represent the 1D histogram in the same bin. The histogram on top shows the number of haloes in every bin, grey for all galaxies and red for galaxies with at least a stellar population that is younger than 5 million years old. Different columns depict different times in the evolution of the universe (as measured by the redshift z). The top row of this figure shows that in all galaxies a high column density around the stars results in a low escape fraction. However, there is a sudden rise in escape fraction at intermediate column densities that can only be explained by splitting the data into two populations: galaxies hosting stellar populations that formed less than 5 million years ago (middle row) and those that do not (bottom row). Young stars produce the majority of ionizing photons over the lifetime of a stellar population. Hence, they are able to penetrate larger column densities. After 5 million years, when the ionizing photon production declines substantially, a similar column density means a smaller escape fraction. This figure explains the circumstances under which galaxies are efficient sources of reionization: a recent burst of star formation in a low-mass proto-galaxy that does not contain enough gas to absorb all the ionizing photons that are produced. This so far over-looked population of galaxies is ubiquitous in the early universe and therefore the most likely source of reionization. The visualisation was done with the help of the astropy package to read in and process the data and the seaborn package to customise the plots. Products -------- - :download:`PDF ` Source ------ .. literalinclude:: plot_scipy_2015.py - :download:`data.zip `